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There is an exploit where a user can deface the login redirect page ( and put ANY VALID HTML there - think about it -

) through one of the fields during registration.
The fix for it is, in user.php, to replace line:
redirect_header($redir,1,sprintf(_US_LOGGINGU, $uname));
With:
$myts = &MyTextSanitizer::getInstance();
$uname_safe=$myts->makeTboxData4Show($uname);
redirect_header($redir,1,sprintf(_US_LOGGINGU, $uname_safe));
Yes I know makeTboxData4Show is deprecated, use the equivalent from the Sanitizer class

Yes I know that this defaces only their own redirect page, but
think about it - it would also allow.....?